1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube having an improved electrode to improve convergence drift of an electron gun.
2. Description of the Related Art
In general, an electron gun, installed at a neck portion of a funnel of a cathode ray tube, emits thermions which excite a phosphor film. The electron gun includes a cathode, a control electrode and a screen electrode forming a triode, and first and second focus electrodes and a final acceleration electrode forming an auxiliary lens and a main lens, respectively. The center of an electron beam passing hole formed at both sides of an output surface of the second focus electrode is offset from the center of an electron beam passing hole at both sides of the final acceleration electrode so that three electron beams converge on a phosphor point.
A predetermined voltage is applied to the electrodes of the electron gun. For instance, an electrostatic focus voltage can be applied to the first focus electrode and a dynamic focus voltage, synchronized with an output signal of a deflection yoke and having the electrostatic focus voltage as a base voltage, can be applied to the second focus electrode.
In the operation of the electron gun for the conventional cathode ray tube having the above structure, when a predetermined voltage is applied to each of the electrodes, an electron lens is formed between each electrode and the thermions, emitted by the cathode and then accelerated, are focused by the electron lenses and land on the phosphor film. Thus, phosphor of the phosphor film is excited so that an image is formed.
Here, when the electron beam is deflected toward the periphery of the phosphor film, as a dynamic focus voltage synchronized with a deflection signal is applied to the second focus electrode, the intensity of the main lens between the second focus lens and the final acceleration electrode is lowered so that convergence of the electron beams located at both sides is not made accurately. As the electrodes of the electron gun are thermally expanded due to the heat generated from the cathode and the deflection yoke, displacement of the electron beam passing holes is made so that a phenomenon of drift of the electron beam is generated.
The drift phenomenon causes inaccurate landing of the electron beam on a phosphor point, thus causing color blurring of an image and lowering the resolution thereof.
To solve the above problems, a conventional electron gun is suggested in U.S. Pat. No. 4,952,186. A method of manufacturing a color cathode ray tube for reduction of convergence drift is disclosed in the patent, in which at least an electrode of the first group electrodes for generating electron beam misconvergence in the first direction is made of material exhibiting a thermal expansion coefficient lower than that of the other electrodes.
However, in the above method, misconvergence due to thermal expansion between the electrodes is not sufficiently corrected.
To solve the above problems, it is an objective of the present invention to provide an electron gun for a color cathode ray tube which can improve the property of convergence by reducing the amount of drift of an electron beam according to thermal expansion of the electrode.
Accordingly, to achieve the above objective, there is provided an electron gun for a color cathode ray tube which comprises a cathode, control electrode and a screen electrode forming a triode, and first and second focus electrodes forming an electron lens, wherein the thermal expansion coefficient of said screen electrode is less than the thermal expansion coefficient of said control electrode.
According to another preferred embodiment of the present invention, there is provided an electron gun for a color cathode ray tube which comprises a cathode, control electrode and a screen electrode forming a triode, and first and second focus electrodes forming an electron lens, wherein the thermal expansion coefficient of said second focus electrode is equal to or less than the thermal expansion coefficient of said control electrode and simultaneously equal to or greater than the thermal expansion coefficient of said screen electrode.
According to another preferred embodiment of th resent invention, there is provided an electron gun for a color cathode ray tube which comprises a cathode, control electrode and a screen electrode forming a triode, and first and second focus electrodes forming an electron lens, wherein the thermal expansion coefficient of said screen electrode is equal to or greater than the thermal expansion coefficient of said control electrode and simultaneously greater than the thermal expansion coefficient of said second focus electrode.
According to still yet another preferred embodiment of the present invention, there is provided an electron gun for a color cathode ray tube which comprises a cathode, control electrode and a screen electrode forming a triode, and first and second focus electrodes forming an electron lens, wherein the thermal expansion coefficient of said second focus electrode is equal to that of said control electrode and simultaneously greater than the thermal expansion coefficient of said screen electrode.